5 The Philippine Journal of Science 



1914 



character of crops that can be grown have been largely worked 

 out by trial. There is no great need of changing the varieties 

 of crops actually grown, but there are tracts of land where 

 these can be much extended and improved and some new crops 

 might be profitably introduced. There are many differences 

 in agricultural practices, and agriculture has attained higher 

 development in some districts than in others. The best methods 

 used by any section should be extended to every other district 

 possessing the same soil and climatic characteristics, special 

 seed selection and rotation of crops should be generally prac- 

 ticed, and the most improved agricultural practices should be 

 introduced among all the farmers. 



With the practice of improved cultural methods and resultant 

 greater yields, a corresponding drain upon the fertility of the 

 soil will follow. Despite the prevalent idea that the fertility 

 of Philippine soils is almost inexhaustible, the practical ex- 

 perience of many farmers is that the land "runs down" after 

 years of continuous cultivation and the crop returns diminish 

 year by year. A careful laboratory investigation and field 

 experiments will usually show the cause and offer a remedy 

 for this. 



Investigators have found that the principal causes of the 

 diminished crop production after years of continuous cultiva- 

 tion are generally the toxic, bacterial, physical, and chemical 

 conditions of the soil. Differences of opinion exist as to which 

 of the above factors predominate, but each determines pro- 

 ductivity to a greater or less extent. These factors are so inter- 

 dependent that a possible treatment for one may affect several 

 or all of the others. 



TOXIC EXCRETA 



Humboldt and De Candolle* and more recently Schreiner and 

 Reed^ have considered the toxic excreta of plants as a factor 

 in soil fertility. The former » suggested the idea of the secre- 

 tion of poisonous excreta by plants as an explanation of the 

 benefit derived from the practice of the rotation of crops, and 

 the latter^ have shown that it is reasonable to believe that toxic 

 excreta of plants inhibit the growth of the same species of plants 

 and thus play a part in diminishing the fertility of lands con- 

 tinuously cultivated in a given crop. These deleterious organic 



^Bull U. S. Dept. Agr., Off. Exp. Sta. (1895), 22, 174. 

 'Bull. U. S. Dept. Agr., Bur. Soils (1907), 40, 5. 

 'Ibid., 37-38; Physiologic Vegetale (1832), 3, 1474. 

 'Loc. cit., 38. 



